Sea wave to electrical energy conversion plant

ABSTRACT

Apparatus for converting the motion of sea wave energy to electrical energy includes one or more float driven linear generators, in which the inertial mass of the float and any linkage means to the linear generator is minimised. In order to generate electrical power consistently upon both the upstroke and downstroke of the float, the moving part of the generator is so sized that its gravitational weight acting upon the float, together with that of the float itself and any intermediate linkage means, is substantially equal to half the total buoyancy of the float.

The present invention relates to the conversion of sea wave energy toelectricity.

A preferred method of obtaining electrical energy from the motion ofwaves is the direct conversion of wave movement to electrical powerusing electrical linear generators. In this arrangement, thereciprocating motion of one or more floats is used to cause relativemovement between the stator and armature of such a generator. The directgeneration of power is thereby realised without the need for any form ofintermediary mechanism, as would be necessary for example when usingrotary electrical generators.

An important factor concerning the generation of electricity from anysource is the efficiency of energy conversion. This is particularlyimportant in the case of capturing renewable energy from sea waves.Because of the high cost of installing the power conversion plant, theoperator must be absolutely certain that the commercial returns will beadequate. It is not possible simply to step up output by burning morefuel, as, obviously, the behaviour of the fuel source (the sea) isoutside the operator's control. Accordingly, in the case of wave powergenerators using linear generators as the energy conversion means, everypossible watt of power must be extracted to ensure an adequate return.For this purpose, it is essential both to ensure that the apparatus isarranged to generate power as consistently as possible, and not to wastethe potential energy available from sea waves on any function subsidiaryto the generation of power.

An example of such a subsidiary function affecting the performance ofmany designs of wave conversion plant using buoyancy chambers, is theexpenditure of wave energy necessary to accelerate the mass itself ofthe buoyancy chamber an any associated apparatus not directly concernedwith the conversion of power. For example, in the case of buoyant hingedraft type constructions, which are used to generate power by employingthe motion of buoyant chambers to drive hydraulic pistons, the mass ofthe raft buoyancy chambers themselves is considerable. As a result ofthis, the upthrust resulting from their buoyancy must be expended bothin accelerating them in order to keep up with the speed of ascendancy ofthe wave acting upon them, and for driving the hydraulic piston.

The disadvantage of this arrangement is readily apparent as thefollowing example shows. Consider a lightweight polystyrene float. Thiswill bob up and down responsively to the action of any wave. A hugeconcrete block, but trapping air so as to have the same overallbuoyancy, cannot possibly do the same, allowing for the basic formulap=ma. The use of the potential energy provided by the wave to lift thepolystyrene float is negligible, and its resulting kinetic energy as itrides up the wave is similarly negligible. It can therefore perform thesingle task of conveying the potential and kinetic energy imparted to itby the wave to the power conversion means, negligible energy being lostin acting upon the float itself. This however, clearly does not apply tothe concrete block. Indeed, if it is too massive, it will not have risenbefore the wave has fallen away.

Also, concerning the effectiveness of using linear generators to convertwave power to electricity, it is important that electrical power isgenerated as consistently as possible, i.e. for a given wave, as evenlyas possible both over the ascent portion of the wave, and the descentportion. Thus, for this purpose, the linear generator should experienceas closely as possible the same upthrust as downthrust during theascending and descending portions of a wave.

The ideal arrangement is one in which the effects of the above describedsubsidiary function are minimised, and at the same time, power isgenerated as consistently as possible.

According to the invention wave energy to electrical power conversionapparatus comprises:

-   at least one linear generator having a stator and an armature which    can be linearly driven relative to the stator to generate electrical    energy and at least one float linked to the armature and which, in    use, is immersed in the sea to be subject to the action of waves to    drive the armature, the float(s), armature and link thereby    constituting a wave-driven mass;-   wherein the weight of the wave-driven mass is substantially equal to    half the upthrust provided by the water displaced by the float(s)    when fully immersed in the water.

Other, optional features of the invention, are defined in thesub-claims.

It should be noted that the invention contemplates more than one floatper generator, and also where there may be more than one generator perfloat. In either case, the mass constraints apply to the combination ofthe float(s)/generator(s)/link(s) as a whole.

Thus, in this arrangement, were there to be no waves i.e. calmconditions were prevailing, on account of the fact that the combinedweights equal half the buoyancy provided by the float were it to befully submerged, the float would float half in—half out of the water(assuming it to be of a symmetrical construction). In the presence ofwaves, during the rise of a wave, (and assuming the mmf resistanceafforded by the generators to the motion of the float is such that it iscompletely submerged during this rising phase), an upwards thrust isimparted to the generator equal to substantially half the weight of thewater displaced by the float. Conversely, on the fall of the wave, andassuming for the same reason the float is hardly in contact with thewater, a downwards thrust due to gravity is imparted to the generatorequal to the combined weights of the assembly, again equal tosubstantially the same value as was experienced on the upstroke.

Thus the linear generators experience substantially consistent upwardsand downwards thrust during the passing of a wave, and thus consistentgeneration of power during both of these phases is achieved. Inaddition, because the mass of the floats and any intermediate linkagemechanism is kept to a minimum, no energy is lost in accelerating anyparasitic weight, other than the necessary mass of the moving componentsof the linear generators themselves, which might otherwise impede thefollowing by the floats of the wave motion.

In other words, all of the available force arising from the presence ofthe float moving upon the waves, and thus the captured energy, isexpended solely in the movement alone of the generators for doing usefulwork (setting aside the comparatively small amount of energy necessaryto accelerate any intermediate linking means, and the lightweight floatitself). At the same time, on account of the combined weights of thegenerator, float(s) and linkage means, the motion of the float isoptimised for the consistent generation of power.

This is an important distinction over prior art disclosures, includinglinear generator wave power devices, in which conscious consideration isnot given to the generation of power as consistently as possible whileat the same time minimising the energy loss through the use ofassociated structures as light as possible.

By way of introduction to an embodiment of the invention describedbelow, many other forms of wave energy devices cannot, by their veryconstruction, make optimum use of the sea area available in which theyoperate. For example, in the case of the type of device in whichbuoyancy chambers are affixed to the far ends of swivelling arms foroperating hydraulic pistons, the opposite ends of the arms being pivotedwithin a tower mounted on the sea bed containing hydraulic pistons andother components, a considerable sea area is monopolised by the arms andthe tower itself. The sea area cannot therefore be used to do usefulwork. In an ideal world, and to obtain the maximum energy from a givenarea of the sea, as many floats as is possible should be operativewithin the waves, without of course disrupting their natural flow andthus effectiveness. It can be envisaged that a honeycomb arrangement offloats would provide an ideal solution.

Therefore, in an embodiment of the invention, the disposition, size andnumber of linear generators operated upon by the float/floats is suchthat the average horizontal area occupied by them does not exceed to anymaterial extent the horizontal area occupied by the float(s) and anyperimeter space surrounding the float for the effective operation andmotion thereof. In this arrangement, the generators do not thereforeoccupy any space greater than that of their associated floats, and thusas many floats can be juxtaposed side by side, or in any otherfavourable arrangement, as is possible. Thus, for any given sized seaarea, and thus size and cost of associated support structure for housingthe linear generators, the maximum power may be generated and thereforethe greatest financial return obtained for initial capital outlay.

In a feature of the invention, in order to enhance the captivation ofwave energy, the flotation chambers are equipped with one or morepaddles immersed in the sea, the planar axes of the paddles beingarranged to be substantially parallel to the sea surface, thearrangement being such that the float and paddles act in combination toforce movement of the armature of the generator relative to its stator,the float by means of its buoyancy, and the paddle, or paddles, by meansof their resistance to the motion of the seawater. Furthermore, theplanar surfaces of the paddle against which the rising and falling waterpresses, may be so contoured as to provide as much resistance aspossible to the motion of the water, and therefore to receive thegreatest counter thrust.

In a further feature of the invention, relating to the shape of thefloats, their profile is optimised such as to provide the maximumpossible buoyancy while offering minimal resistance to the slightelliptical horizontal movement experienced by waves as they rise andfall. Thus minimal sideways forces are communicated to any supportstructure supporting the linear generators. This profile may be, forexample, in the shape of a ‘flying saucer’.

In one embodiment of this invention, the stator of a linear motor ispartially immersed in the sea, and is held perpendicular to the sea bedby a weight resting on the sea floor, or by other permanent means. Thecoaxial armature for traversing up and down the rod, and the generationof electricity, is directly fixed to, or is integral to, the flotationchamber—and paddles—which are also coaxial with the rod and free totravel therealong. (Note, for the purpose of clarifying the terminologyused throughout this application, by armature is meant that part of thelinear generator which is caused to move by the float.)

The gravitational weight of the armature, along with that of theflotation chamber is so predetermined that in use substantially half ofthe flotation chamber would protrude above water during calm conditions.Thus, in wave conditions, as waves ascend, its natural buoyancy raisesthe assembly to generate electricity, and as the waves fall, the weightof the assembly causes it likewise to fall, again generatingelectricity. To suit local conditions, in a feature of this embodimentof the invention, rather than using one coaxial flotation chamber permotor, one large floatation chamber may be linked by articulated jointsto several generators.

In an alternative arrangement relating to this embodiment of theinvention, the linear generator, or generators, rather than beingimmersed in the sea, is/are instead mounted within a supporting cageabove sea level. The generator may be protected from sea spray and thewind by suitable cowlings. In this arrangement, the flotation chamberand paddles—moving in response to the undulation of the sea waves—areconnected by push rods, or other mechanical means, to the moving part ofthe housed generator(s). Thus, the aggressive and inhospitable aspect ofgenerating power from waves is confined solely to the field (ocean)replaceable flotation and paddle components. In an aspect of this formof arrangement, in order to cope with the very considerable variationsin wave height arising from tidal movement affecting power plantslocated near the sea shore, means are provided within the cage to varythe height, relative to the sea bed, of the fixed part of the generator,in accordance with the mean height of the waves.

Referring now to an aspect of the invention concerned with howgeneration of electrical power is optimised for any given prevailingwave condition, control means are used to regulate the effective loadimpedance presented to the generators in accordance with the strength ofthe prevailing wave motion, the regulation being such as to ensure thatthe electromagnetic damping of the motion of the generators, as theygenerate electricity, is always such as to optimise the generation ofpower. By way of explanation, if the generator is either over or underdamped, it will fail to respond in the optimal manner to movement of thewaves, inasmuch that its frequency response will not enable sympatheticmotion corresponding to that of the waves.

The invention will now be described with reference to the accompanyingdrawings in which:—

FIG. 1 is a schematic representation of a wave generator of theinvention;

FIGS. 2 a and b show the principal components of FIG. 1, includingvectors showing their gravitational weights;

FIG. 3 shows an arrangement in which the generators are mounted abovesea level in a cage;

FIG. 4 shows a multiplicity of generators mounted above and within theconfines of the horizontal surface area of a float; and

FIG. 5 shows typical electrical current waveforms generated by the wavemovement, and control circuitry for optimising the use of the availablewave power in any set of prevailing conditions.

Referring to FIG. 1, wave energy to electrical energy conversionapparatus is depicted immersed in the sea. The apparatus comprises afloat driven linear generator, the stator of which comprises a fixed rod10, which houses a sequence of permanent magnets. The rod is embedded,at its lowest extremity, in a concrete block, 11. The block itself isanchored to the sea bed—shown generally at 12- to avoid drifting.

The armature of the generator 13 comprises a cylindrical housing inwhich is embedded a series of coils. Coaxially surrounding the armature,and affixed thereto, is an annular flotation chamber 14; the fixture ofthe chamber 14 to the armature, at 100, constitutes a link by means ofwhich motion of the chamber 14 drives the armature. The float is made ofa construction which is as light as possible. This is in order to ensurethat its weight is negligible in comparison with that of the armature ofthe generator, and therefore that the wave energy present is expendedusefully on generating electrical power rather than accelerating anyundue mass of the float itself, and/or restricting the assembly fromfollowing the wave motion. Located at the upper and lower surfaces ofthe armature are bearing bocks 15 and 16, for guiding the armaturecoaxially up and down the stator. Annular paddles 17, are also affixedto the flotation chamber. The paddles are contoured in order to offer asmuch resistance as possible to vertical movements of the sea water, seeinset diagram at 17 a. The size and/or length of the armature of thelinear generator, and thus its weight, is so selected that its weight,combined with that of the float, is such as to counteract by half thetotal upthrust afforded by the volume of water that would be displacedby the float were the float to be submerged. This is shown more clearlywith reference to FIGS. 2 a and B. The weight W1 of the linear generatorarmature 13, combined with the weight W2 of the float 14, i.e. W1+W2, isarranged to equal substantially half the upthrust U1 of water displacedby the float were it to be fully submerged.

The action of the apparatus is as follows. As a wave arrives, thenatural buoyancy of the flotation chamber causes the whole assembly torise. This is assisted by the pressure of the rising water actingagainst the paddles 17. Thus relative motion arises between the armatureand stator of the linear generator and alternating current is generatedwithin the coils of the generator, the amplitude and frequency of whichdepend upon the vigour of the wave motion. The current is conducted to ashore station by a suitably armoured flexible cable, 18. (Note, means,not shown, are present to prevent rotation of the assembly and thereforeunwanted tensioning of the cable.)

Once the wave has reached its zenith, and begins to fall, the weight ofthe assembly causes the same also to fall. Power again is generated asthe armature traverses the stator. Because the upthrust experienced bythe generator is substantially the same as the weight of the assembly,electricity is generated reasonably consistently both upon the rise andfall phases of the wave. There is some natural phase lag between theascending of the assembly relative to the waves, and its fall, due tothe natural damping effect of the electromotive force generated. As willbe hereinafter described in more detail, the load impedance presented tothe generator, and the overall weight of the moving assembly, is soselected as to optimise generation for any particular wave condition.

The apparatus of the invention thereby generates electrical energyconsistently by the simple expedient of using an elongate linear motorhaving an armature of appropriate weight acting in reverse as a wavepowered linear generator. In addition, owing to the lightweightconstruction of the float itself, the available sea wave energy isexpended in causing the relative motion of the generator armature to itsstator, rather than being expended also on the mass of the float itself.

As an alternative to the immersing of the generator in the sea, and/orwhen generation is to be effected at a sea depth where it is impracticalto use the arrangements of FIG. 1, an alternative method of mounting thegenerator may be employed with many practical advantages. Referring toFIG. 3, rather than the generator being immersed in the sea, it isinstead mounted within a cage 19, which in this illustration, itselfrests on and is anchored to the sea bed. (Alternatively, in the case ofoperation in deep waters, the cage may be supported on the sea surfaceby separate buoyancy chambers, and moored by anchor to the sea bed.) Inthis arrangement, the moving part of the generator is the armature 20.(Note, for the purpose of clarifying the terminology used throughoutthis application, by armature is meant that part of the linear generatorwhich is caused to move by the float.) For the purpose of economy ofconstruction, not all of the armature contains permanent magnets. On thecontrary, the armature tube is only filled with magnets in the verticalactive central portion thereof which, in normal use, traverses past thestator. The portion 20 a and 20 b which extend respectively downwards tothe float 21, and upwards through the guide rollers 22, are insteadfilled with a material of appropriate durability and structuralstrength. As can be seen from the figure, the stator 23, is mounted on aplatform 24, within the cage. As the float and paddle combination 21 iscaused to ascend/descend by the undulation of the waves, so the armatureis moved through the fixed stator to generate power. The armature isguided both above and below the stator by the rollers, 22. Both thefloat and the portions 20 a and 20 b of the armature rod 20, are made ofmaterial of as light weight as possible, commensurate with adequatestructural strength, to ensure that the available sea energy is expendedon useful generation. The portion 20 a of the armature may be considereda link which mechanically connects the float 21 to the generative partof the armature 20 in the region of the stator 23.

As is well known, there can be significant variations of the height ofthe sea close by the sea shore due to tidal motion. Thus, according tothe time of day, the height of the peaks and troughs of any ‘given size’wave may vary substantially relative to the sea bed. This must beaccommodated in the case of near shore location of any of thearrangements herein disclosed, by the use of a sufficiently longarmature rod. However, filling an extended rod along its whole lengthwith magnetic material, so that an ‘active portion’ is always presentedto the stator, entails undue expense. To cope with this situation, thestator may be situated on the vertical movable platform 24. The heightof the platform may be adjusted by detection means, (not shown), to varywith the mean height of the waves, i.e. the average tidal level, by leadscrew actuators, 25. The stator may be cooled by sea water pumped arounda cooling jacket surrounding the same.

Referring to FIG. 4, a balance is achieved between the length L of thelinear generators 13 and the total number N used for any given float,such that, while achieving the necessary mmf resistive force, the totalhorizontal area A1 occupied by them falls within the equivalenthorizontal area A2 occupied by the float, as shown. By this means,floats can be closely juxtaposed in order to make optimum use of the seaarea in which they are immersed, and thus achieve the largest possibleconversion of sea wave energy to electrical energy for a given area.This additionally provides the maximum return in terms of revenue foreach kilowatt hour generated, for a given capital outlay on supportstructures.

For any given sea condition, which may vary from a light swell to araging storm, it is important to ensure that the moving part of thegenerator faithfully follows the movement of the waves. For example,should the armature be feeding a short circuit, its motion would beexcessively damped, and the float/paddle combination would be unable tofollow the waves in an optimal fashion. Similarly, in a storm, were thegenerator to be feeding effectively an open circuit, the assembly mayrise too easily in response to an approaching wave, and, under its ownmomentum, overshoot the crest thereof. Therefore means are necessary toensure the load impedance is suitably adjusted for any given wavepattern. Referring to FIG. 5, AC currents generated within the coils ofthe generator, shown symbolically by way of example at 26 and 27, arefirst rectified by bridge rectifiers 28 and 29. The resulting DCcurrents are then fed into storage means 30. The storage means assistsboth in producing a steady DC level, and for ensuring a constant supplyof energy whether a storm is present or an intervening calm. An inverter31, is used, via a transformer 33, to supply alternating current to theelectricity distribution system. The effective impedance of the inverteris dynamically adjusted by detection means 32 which itself is responsiveto the form of current the generators are attempting to generate, inorder to optimise generation of power for any prevailing wave condition.Thus the criteria outlined above for ensuring optimal matching of thegenerating capacity of the generator, with that of the motion of thewaves, is permanently self-optimised. Other control circuitry means,(not shown), are used—as is customary in generating stations—to ensurethe phase angle of the generated currents is correct for the addition ofpower to the distribution system.

An additional feature of the invention, which indeed can be applied toall the arrangements described herein, is that the permanent magnetswithin the rod of the generator may, according to their position alongthe rod, be made of permanent magnetic materials of varying fieldstrengths and therefore cost, such that at the middle of the stator,where motion of the armature will be at its greatest, are located thestrongest magnets, and at the extremities of the rod, weaker magnets.This arrangement can thereby also be used to match the predominant waveconditions, as well as economising in the cost of magnets used.

Numerous variations of the above will be apparent to those skilled inthe art.

1. An apparatus of wave energy to electrical energy power conversioncomprising: at least one linear generator having a stator and anarmature which can be linearly driven relative to the stator to generateelectrical energy and at least one float linked to the armature by linkmeans and which, in use, is immersed in the sea to be subject to theaction of waves to drive the armature, the at least one float, the linkmeans and the armature thereby constituting a wave-driven mass; whereinthe weight of the wave-driven mass is substantially equal to half theupthrust provided by the water displaced by the at least one float(s)when fully immersed in the water, and wherein the contribution to theweight of the wave driven mass of the at least one float and the linkmeans is negligible compared with that of the armature; and furthercomprising control means used to regulate the effective load impedancepresented to the generator or generators in accordance with the strengthof the prevailing wave motion acting upon the at least one float, theregulation being such as to ensure that the electromagnetic damping ofthe motion of the armature of the or each generator as it generateselectricity, is always such as to optimise the generation of power. 2.The apparatus of claim 1, wherein the average horizontal area occupiedby the at least one linear generators does not exceed to any materialextent the horizontal area occupied by the at least one float and anyperimeter space surrounding the at least one float for the effectiveoperation and motion thereof.
 3. The apparatus of claim 1, wherein theat least one float is equipped with one or more paddles, suitablycontoured, to augment the force of the sea waves acting upon the atleast one float.
 4. The apparatus of claim 1, wherein the at least onefloat is so contoured as to minimise any wave latent forces acting uponit, while maximising its buoyancy.
 5. The apparatus of claim 1, whereinthe stator of the at least one linear generator is maintained stationaryand substantially perpendicular to the sea bed, and the armature thereofis affixed directly to the at least one float for traversing the statorin accordance with the motion of the waves acting upon the at least onefloat.
 6. The apparatus of claim 1, wherein the stator of the at leastone linear generator is held in a cage above sea level.
 7. The apparatusof claim 6, wherein the link to the float is a direct extension of thearmature of the at least one generator.